1. Field of the Invention
The present invention relates to an electron mediator (hereinafter, simply referred to as “mediator”) used in a biofuel cell or a biosensor employing a bioenzyme as a catalyst, an electron mediator immobilized electrode, and a biofuel cell using the electrode.
2. Description of the Related Art
A bioelectrochemical fuel cell (hereinafter, referred to as “biofuel cell”) is drawing increasing attention from considerations in safety and cost of fuel cell materials. The biofuel cell employs a biocatalyst such as a microorganism or an enzyme and uses biomass for an anode fuel. Examples of the biomass include hydrogen, methanol, and in addition, glucose, which is very easily handled and which abundantly exists as a source of nutrition for the human beings. The biofuel cell employs a biocatalyst, and hence has a characteristic of allowing operation under mild conditions of a physiological environment such as room temperature, neutrality, and atmospheric pressure. Theoretical electromotive force of a glucose/O2-type biofuel cell of 1.25 V is provided from a difference between a redox potential of the glucose, which is an anode fuel, and a redox potential of the O2, which is a cathode fuel. A redox reaction of glucose using an electrode is very slow in actuality, requiring an enzyme to be used as an electrocatalyst. However, a redox reaction active center of an enzyme is covered with a polymer protein. Thus, electrolytic oxidation of the glucose would not be possible without using a mediator carrying out electron transfer between an enzyme and an electrode.
Possibility for miniaturization of such a biofuel cell is expected, and the biofuel cell may be applied in the field of medicine for an implanted biological device which can operate on its own such as a nanorobot, a micropump, a pace maker, or a miniature glucose sensor.
Examples of a mediator for such a biological device usually included a mediator employing an osmium complex for a biofuel cell and a mediator employing an iron cyanide complex for a glucose sensor. However, those mediators had problems in safety and cost.
Further, JP 2000-133297 A discloses a mediator for a biofuel cell composed of a compound having a thionine skeleton and a 2-hydroxy-1,4-naphthoquinone skeleton. The mediator has the compound immobilized on an electrode for reducing an amount of the mediator used so that diffusion of the mediator in an electrode solution does not become rate controlling.
Immobilization of the mediator on an electrode involves subjecting the electrode to plasma treatment and further subjecting the electrode to treatment with 3-aminopropyltriethoxysilane, to thereby form an electrode with an amino group introduced to an electrode surface. The immobilization further involves: immobilizing long-chain alcohol on a carbon electrode surface through dehydration condensation; and immobilizing the immobilized long-chain alcohol and 2-hydroxy-1,4-naphthoquinone through dehydration condensation.
Such immobilization involves complicated operations, and has had practical problems. The immobilization involved monomolecular layer modification, and hence an amount of the mediator immobilized became small and thus an amount of an enzyme immobilized was also restricted, resulting in limited reactivity.
Further, a non-patent document (K. Miki, T. Ikeda, S. Todoroki, and M. Senda, Analytical Sciences, 1989, 5(3), 269) discloses 2-methyl-1,4-naphthoquinone (VK3) as a mediator. The mediator and NAD+ are mixed with a carbon paste constituting an electrode, and diaphorase and dehydrogenase are injected to an electrode surface. Then, the electrode surface is covered with a nylon film to be used as a biocatalyst electrode.
The biocatalyst electrode has the mediator merely sealed in the paste physically. Therefore, the biocatalyst electrode has had practical problems in that a mediator molecule elutes from the electrode surface in long-term use to degrade performance as an electrode.